Progabide (ADD 67003) has been shown to be an effective anticovulsant in various animal models of epilepsy. It is metabolized to 4-((4-chlorophenyl)(5-fluoro-2-hydroxy phenyl) methylene) butanoic acid. This acid metabolite has anticonvulsant activity similar to that of Progabide. Progabide is now undergoing a NINCDS-sponsored clinical efficacy trial. Plasma level measurements of both parent drug and active metabolite are an integral part of these studies. A high pressure liquid chromatographic (HPLC) method has been developed that is sensitive enough to simultaneously quantitate both drug and metabolite in plasma of epileptic patients receiving orally administered drug. This method uses an electrochemical detector for specificity and sensitivity. Both drug and metabolite can be quantitated to levels of 10 ng/ml of plasma and the assay is presently being used in the NINCDS clinical studies.